Aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN) are advanced materials of emerging importance for a number of applications in the microelectronics, optoelectronics, aerospace and other industries (L. M. Sheppard, Ceramic Bull., 69:1801-1812 (1990), xe2x80x9cAluminum Nitride: A Versatile but Challenging Materialxe2x80x9d). AlN is an ideal heat transfer medium for high power electronic devices and multichip modules because of its favorable thermal and electronic conductivity. It is also highly refractory and mechanically strong, making it useful for high temperature processes and abrasion and corrosion protection of surfaces. Depending on the specific application, bulk powder or a thin film may be required.
Bulk powders are used to press shapes, such as crucibles or rods and other structures. Popular methods of preparing bulk powder AlN comprise carbothermal reduction of Al2O3 in the presence of nitrogen and direct nitridation of metallic aluminum and reaction of AlCl3 with NH3. (L. M. Sheppard, 1990, supra). Each method has certain advantages and drawbacks. Carbothermal reduction proceeds at 1100xc2x0 C., with removal of unreacted carbon at 600-700xc2x0 C. in dry air and further heating at 1400xc2x0 C. in vacuum. Residual carbon and oxide impurity in the final material can be a problem for batch quality.
Direct nitridation (1200xc2x0 C.) suffers from incomplete conversion of starting materials and coalescence of metallic aluminum. Gas phase approaches, such as reaction of AlCl3 with NH3, are inherently low yield. Bulk polymeric aminoalane AlCl3 precursors have been prepared electrochemically by anodization of an Al electrode in liquid NH3 at xe2x88x9270xc2x0 C. (C. B. Ross et al., Chem. Mater., 3:768-771 (1991), xe2x80x9cElectrochemical Synthesis of Metal Nitride Ceramic Precursors in Liquid Ammonia Electrolyte Solutionsxe2x80x9d). Thermal decomposition of polyaminoalanes prepared by direct reaction of neat N2H4 with AlCl3 has been reported (W. G. Paterson and M. Onyszchuk, Can. J Chem., 41:1872-1876 (1963), xe2x80x9cThe Interaction of Hydrazine with Boron and Aluminum Halidesxe2x80x9d).
Thin films commonly needed in microelectronic or surface protection applications are usually prepared by thermal decomposition of a gas phase precursor at high temperature by chemical vapor deposition (CVD). A number of these precursors have been reported including [(CH3)2AlNH2]3(L. V. Interrante et al., J Electrochem. Soc., 136:472-478 (1989), xe2x80x9cPreparation and Properties of Aluminum Nitride Films Using an Organometallic Precursorxe2x80x9d), Al(C2H5)3+NH3 (A. A. Adjaottor and G. L. Griffin, J. Am. Ceram. Soc., 75:3209-3214 (1992), xe2x80x9cAerosol Synthesis of Aluminum Nitride Powder Using Metalorganic Reactantsxe2x80x9d) and basic aluminum chloride/glucose (N. Hashiomoto et al., J Am. Ceram. Soc., 75:2098-2106 (1992), xe2x80x9cSintering Behavior of Fine Aluminum Nitride Powder Synthesized from Aluminum Polynuclear Complexesxe2x80x9d). The common problem of CVD with these materials is that they contain carbon and/or oxygen in their structures, which then contaminates the final product as oxide or carbide. This has implications for thermal conductivity of the material, which depends on impurity levels. CVD methods can be quite complex as well, requiring optimization of formation of the precursor in the gas phase in addition to optimization of the CVD reaction itself.
Processes using molten salts to produce metal nitrides have been reported; however, these reactions are run at temperatures higher than 200xc2x0 C. See, e.g., U.S. Pat. No. 4,029,740 to Ervin, Jr. for xe2x80x9cMethod of Producing Metal Nitrides.xe2x80x9d
Chloroaluminate molten salts combined with alkali metal chlorides to provide Lewis acid-base neutrality are described in U.S. Pat. No. 5,096,789 . This patent does not teach the desirability of acidic melts nor the preparation of nitrides.
Ambient temperature molten salts of chloroaluminates and 1-butylpyridinium chloride and 1-ethyl-3-methylimidazolium chloride are described in Osteryoung, R. A., (1987) xe2x80x9cOrganic Chloroaluminate Ambient Temperature Molten Salts,xe2x80x9d in Molten Salt Chemistry, G. Mamantov and R. Marassi (eds): 329-364 . Electrochemical and spectroscopic behavior of certain hydrocarbons in AlCl3-1-butylpyridinium chloride or 1-ethyl-3-methylimidazolium chloride molten salt systems have been studied as reported in Robinson, J. and Osteryoung, R. A. (1979), xe2x80x9cAn Electrochemical and Spectroscopic Study of Some Aromatic Hydrocarbons in the Room Temperature Molten Salt System Aluminum Chloride-n-Butylpyridinium Chloride,xe2x80x9d J. Amer. Chem. Soc. 101(2):323-327; and Uribe, F. and Osteryoung, R. A. (1988), xe2x80x9cElectrochemical and Spectroscopic Studies of 1,4-Benzoquinone in Ambient Temperature Chloroaluminate Ionic Liquids,xe2x80x9d J. Electrochem. Soc. 135(2):378-381.
Plating baths for electrodeposition of aluminum using molten salt baths comprising aluminum halide and a quaternary ammonium salt comprising an alkylimidazolium halide are disclosed in U.S. Pat. No. 4,904,355 to Takahashi for xe2x80x9cPlating Bath for Electrodeposition of Aluminum and Plating Process of Making Use of the Bath.xe2x80x9d U.S. Pat. No. 5,135,825 to Mori et al. for xe2x80x9cMethod for Producing Ambient Temperature Molten Salt Consisting of Certain Pyridinium and Imidazolium Halides and an Aluminum Trihalidexe2x80x9d discloses the use of an inert solvent having a low boiling point for making a molten salt. U.S. Pat. 5,543,522 to Kawahara et al. for xe2x80x9cProcess for Preparing an Ambient Temperature Molten Salt Using Thionyl Chloridexe2x80x9d discloses ambient temperature molten salts for electroplating comprising aluminum halide and onium halide together with thionyl chloride. U.S. Pat. No. 5,552,241 to Mamantov et al. for xe2x80x9cLow Temperature Molten Salt Compositions Containing Fluoropyrazolium Saltsxe2x80x9d discloses mixtures of metal halides and fluoropyrazolium salts for use in electrochemical cells. The use of such salts in processes for forming metal nitrides, however, is not suggested in these patents.
Molten salts of aluminum, gallium or indium with hydrocarbyl-saturated onium ions are disclosed in U.S. Pat. No. 4,764,440 to Jones and Blomgren for xe2x80x9cLow Temperature Molten Compositions.xe2x80x9d These salts are molten below about 100xc2x0 C. U.S. Pat. No. 4,883,567 to Verbrugge et al. for xe2x80x9cMethod of Plating Metallo-gallium Filmsxe2x80x9d discloses the use of a room-temperature melt consisting of GaCl3-dialkylimidazolium chloride and a salt of a metal to be codeposited for electrodeposition of gallium-arsenic gallium-antimony or gallium-aluminum. U.S. Pat. No. 5,463,158 to Goledzinowski et al. for xe2x80x9cOligomerization of Low Molecular Weight Olefins in Ambient Temperature Meltsxe2x80x9d also discloses aluminum and gallium halide and certain organic halides (containing N-heterocyclic rings and substituted onium ions) used to form molten salts in catalytic systems. Molten salts of gallium with certain organic halides have also been reported in S. P. Wicelinski et al., J. Electrochem. Soc., 134:262-263 (1987), xe2x80x9cLow Temperature Chlorogallate Molten Salt Systems.xe2x80x9d These salts apparently display chlorogallate equilibria analogous to that of the chloroaluminates. The GaCl3-MEIC (MEIC, 1-ethyl-3-methylimidazolium chloride, also known as EMIC) and GaCl3-BPC systems (BPC, N-butylpyridinium chloride) are liquid at ambient temperature over a wide range of compositions (S. P. Wicelinski et al., supra) but require the synthesis of the organic chloride salt (J. S. Wilkes et al., Inorg. Chem., 21:1263-1264 (1982), xe2x80x9cDialkylimidazolium Chloroaluminate Melts: A New Class of Room-Temperature Ionic Liquids for Electrochemistry, Spectroscopy and Synthesisxe2x80x9d; S. D. Jones and G. E. Blomgren, J. Electrochem. Soc., 136:424-427 (1989), xe2x80x9cLow-Temperature Molten Salt Electrolytes Based on Aralkyl Quaternary or Ternary Onium Saltsxe2x80x9d).
A dialkylimidazolium chloride:InCl3 molten salt that melts below 45xc2x0 C. for a particular basic composition, 45:55 mole % InCl3:RCl, used to electrodeposit mixed InSb films has been reported (M. K. Carpenter and M. W. Verbrugge, U.S. Pat. No. 5,264,111, xe2x80x9cMethods of Making Thin InSb Filmsxe2x80x9d); however, to our knowledge ambient temperature molten salts of the form RCl:InCl3 have not been prepared.
Reaction of acidic melts comprising Group III metal halides and organic halide components with ammonia or hydrazine do not appear to have been previously reported.
It is an object of this invention to provide methods for the preparation of precursors to III-V metal compounds (compounds such as AlN having a Group III metal combined with a Group V element) and room-temperature molten salts and methods for solvent-free preparation of such precursors, and also to provide methods for producing such precursors as stable, melt-insoluble compounds that are easy to isolate.
All publications referred to herein are hereby incorporated by reference to the extent not inconsistent herewith.
Processes are provided for preparation of precursors of III-V compounds, i.e., compounds formed of a Group III metal in combination with a Group V element, preferably nitrogen, wherein said precursors comprise M, X and V, where M is a Group III metal selected from the group consisting of boron, aluminum, gallium, and indium, X is a halide, preferably chloride or bromide and more preferably chloride, and V is a Group V element selected from the group consisting of nitrogen, phosphorous, arsenic, antimony and bismuth. These precursors are easily converted to MV compounds, preferably Group III metal nitrides. Such compounds, especially the nitrides, are useful in the formation of thin films to provide coatings for electronic components and powders for pressing into shapes such as rods, discs and tubes. The precursors are generally in the form of molecules or complexes comprising the Group III metal, nitrogenous moieties or other Group V moieties, and halides, and reference to M, V and X herein refer to these components as they exist in combined rather than elemental form.
The processes comprise: (a) preparing an active halide ion comprising M by forming a mixture of MX3 (a halide of M) with RXxe2x80x2 (an organic halide) where R is an asymmetrical organic cation and Xxe2x80x2 is a halide which may be the same or different from X (the halide of M), wherein said mixture comprises a molar ratio of MX3 to RXxe2x80x2 to said organic halide such that said mixture forms a salt comprising an active halide ion comprising M, X and Xxe2x80x2 which is molten at about 200xc2x0 C. or less; and (b) contacting said active halide ion with a V-containing material whereby said precursor is formed.
An active halide ion comprising M is a reactive Lewis Acid such as a Group III metal halide having excess halide, M2Cl7xe2x88x92, e.g., B2Cl7xe2x88x92, Al2Cl7xe2x88x92, Ga2Cl7xe2x88x92, In2Cl7xe2x88x92, or Al3Cl10xe2x88x92, capable of reacting with V-containing compounds as defined herein, e.g., ammonia or hydrazine, to form stable products which yield III-V compounds upon thermal decomposition.
R is an asymmetrical organic cation preferably containing nitrogen, sulfur or phosphorus, preferably nitrogen. Preferably, R comprises a ring structure, either saturated or unsaturated. More preferably, R comprises an aryl or N-heterocyclic group which is preferably phenyl, pyridyl or imidazoyl. Such rings may be substituted with alkyl, aryl or aralkyl groups. The term xe2x80x9casymmetricxe2x80x9d with respect to the organic cation refers to molecules (ions) having no plane of symmetry.
Preferred RXxe2x80x2 are asymmetrical molecules selected from the formulae: 
where R1, R2 and R3 are independently H, alkyl or aryl.
Preferably RXxe2x80x2 is 1-butylpyridinium chloride (BPC), 1-ethyl-3-methylimidazolium chloride (MEIC or EMIC) or trimethylphenylammonium chloride (TMPACl). TMPACl is commercially available.
A suitable V-containing compound is any such compound capable of providing a V ion having an unshared pair of electrons in the molten salt. Preferably V-containing compounds are nitrogen-containing compounds not containing carbon or oxygen, such as ammonia, hydrazine or an azide such as NaN3 or other Group I metal azide. Other nitrogen-containing compounds such as alkyl-substituted hydrazines, aliphatic and aromatic amines and other nitrogen-containing species such as cyclic aromatic compounds with one or more nitrogen atoms may also be used, but they are less desirable because they do not produce precursors which yield as pure a nitride product upon heating as nitrogen-containing compounds not containing carbon or oxygen.
The molar ratio of MX3 to RXxe2x80x2, as well as the specific M, X and R used, helps determine the temperature at which the mixture is molten, as is known to the art or easily ascertainable by those skilled in the art following known analytical procedures for melting point determinations. Equimolar amounts of AlCl3 and RCl form a neutral melt comprising mostly AlCl4xe2x88x92 which may be liquid at room temperature. Acidic melts also containing Al2Cl7xe2x88x92 are formed by adding excess AlCl3. These acidic melts may be liquid at room temperature. The molten salts useful in this invention are low temperature molten salts which are liquid or partly liquid at temperatures below about 200xc2x0 C. Preferred molten salts useful in this invention are molten salts which are liquid or partly liquid at temperatures below about 100xc2x0 C., more preferably below about 45xc2x0 C. and most preferably at ambient temperature or below. xe2x80x9cLiquidxe2x80x9d salts may be formed by mixing two solids or by heating a single component or mixture of two components above its melting point.
A molar ratio of MX3 to RXxe2x80x2 of about 3 to 1 is generally preferred herein. Molar ratios of 2.5:1 and 2:1 are also useful in this invention as are the whole continuum of values between x:1 where 1 less than xc3x97xe2x89xa63. Molar ratios higher than3:1 MX3:RXxe2x80x2 resulting in mixtures which are partly liquid at low temperatures are also suitable for use in this invention.
The process for making precursor materials of this invention is carried out using a molten salt mixture at an initial temperature below about 200xc2x0 C., preferably about 100xc2x0 C. or less, more preferably at about 45xc2x0 C. or less, and most preferably at ambient temperature or less, in the absence of a solvent. If the molten salt is only partly liquid at the initial reaction temperature, so long as the liquid portion is acidic, the reaction should proceed, with solid MX3 in the reaction vessel going into solution as precursor is formed until the liquid becomes neutral. The reaction may be exothermic, further shifting the reaction equilibrium toward formation of precursor material. Water should be excluded from the reaction vessel.
The precursor produced by the above process is decomposed, preferably by applying heat, to form the corresponding III-V compound. To decompose the precursor, temperatures of between about 200xc2x0 C. and about 1000xc2x0 C. may generally be used, preferably between about 800xc2x0 C. and about 900xc2x0 C. The precursor material may be heated to vaporize the precursor and deposit the III-V product as a protective film on a desired surface, e.g., that of an aerospace component such as a turbine blade by thermal decomposition. Alternatively, the III-V product may be condensed to form a powder which may be pressed into desired shapes such as crucibles or rods and other structures, preferably with application of heat to anneal the compound, e.g., about 1000xc2x0 C. to about 1300xc2x0 C. Use of ammonia as a preferred V-containing material produces precursors especially suitable for decomposition and vaporization of Group III metal nitrides suitable for chemical vapor deposition (CVD) of nitride coatings.
When hydrazine or azide is used as the V-containing material, the precursor forming reaction proceeds at temperatures comparable to those required for ammonia. Hydrazine, which is liquid at room temperature, may be injected via a cannulus into the molten salt and forms a solid, insoluble precursor product with aluminum chloride suggestive of a polymer which can be applied to coat the surface of an object by painting or dipping, or which can be directly cast into desired forms such as rods, etc.